Strong incentives exist for the development of efficient processes for the separation of oxygen from gas mixtures, such as air. Low-cost production would enhance the availability of pure oxygen for a variety of industrial applications including its use in high efficiency combustion processes. There is significant potential for the application of solid state catalytic membranes to oxygen separation. This technology is presently limited by the ceramic materials that are available. New ceramic materials that exhibit higher oxygen flux and improved mechanical and chemical stability in long term operation for use in membrane reactors are of significant interest in the art.
This invention relates to mixed metal oxide materials that are particularly useful for the manufacture of catalytic membranes for gas-phase oxygen separation processes. Oxygen-deficient oxides of this invention are derived from brownmillerite materials which have the general structure A2B2O5. The materials of this invention maintain high oxygen anion conductivities at relatively low membrane operating conditions ranging from about 700xc2x0 C. to 900xc2x0 C. The metal elements at the B-site in the brownmillerite structure are selected to provide mixed ion- and electron-conducting materials and, particularly, to provide materials that conduct oxygen anions and electrons. The materials of this invention have the general formula:
AxAxe2x80x2xxe2x80x2Axe2x80x32-(x+xxe2x80x2)ByBxe2x80x2yxe2x80x2Bxe2x80x32-(y+yxe2x80x2)O5+z
where:
x and xxe2x80x2 are greater than 0;
y and yxe2x80x2 are greater than 0;
x+xxe2x80x2 is less than or equal to 2;
y+yxe2x80x2 is less than or equal to 2;
z is a number that makes the metal oxide charge neutral;
A is an element selected from the lanthanide elements and yttrium;
Axe2x80x2 is an element selected from the Group IIA elements;
Axe2x80x3 is an element selected from the f block lanthanides, Be, Mg, Ca, Sr, Ba and Ra;
B is an element selected from the group consisting of Al, Ga, In or mixtures thereof; and
Bxe2x80x2 and Bxe2x80x3 are different elements and are independently selected from the group of elements Mg or the d-block transition elements.
The lanthanide metals include the f block lanthanide metals: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Yttrium has properties similar to the f block lanthanide metals and is also included herein in the definition of lanthanide metals. A is preferably La or Gd, with La more preferred. Group IIA metal elements of the Periodic Table are Be, Mg, Ca, Sr, Ba, and Ra. The preferred Group IIA elements for the Axe2x80x2 element of the materials of this invention are Ca, Sr and Ba and Sr is most preferred. The more preferred B elements are Ga and Al, with Ga more preferred. The d block transition elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. Preferred Bxe2x80x2 and Bxe2x80x3 elements are Mg, Fe and Co, with Fe and Co being more preferred as Bxe2x80x2 and Bxe2x80x3, respectively.
Mixed metal oxides in which Bxe2x80x2 and Bxe2x80x3 are Fe and Co are particularly preferred for membranes having high oxygen flux rates.
Mixed metal oxides in which Bxe2x80x2 and Bxe2x80x3 are Fe and Mg are also preferred for membranes having high oxygen flux rates.
The value of z in the above formula depends upon the values of x, xxe2x80x2, y and yxe2x80x2 and the oxidation states of the A, Axe2x80x2, Axe2x80x3, B, Bxe2x80x2 and Bxe2x80x3 elements. The value of z is such that the mixed metal oxide material is charge neutral. In preferred materials, 0 less than z less than 1.
Preferred stoichiometries for materials of this invention of the above formula are those in which x is about 0.1 to about 0.6, and xxe2x80x2 is about 1.4 to about 1.9, and where in addition x+xxe2x80x2 is about equal to 2. When x+xxe2x80x2 is equal to 2, the mixed metal oxide contains only A and Axe2x80x2 metals. More preferred are materials in which x is about 0.2 to about 0.5 and xxe2x80x2 is about 1.5 to about 1.8. Also preferred are those materials of the above formula where y is about 0.3 to about 0.9 and yxe2x80x2 is about 0.6 to about 1.8. More preferred materials have y equal to about 0.5 to about 0.8 and yxe2x80x2 equal to about 1.0 to about 1.4. Preferred materials have y+yxe2x80x2 equal to about 1.4 up to about 2.0. More preferred materials have y+yxe2x80x2 equal to about 1.6 to about 1.9.
In specific embodiments, mixed metal oxide of this invention include those of the above formula wherein Bxe2x80x3 is Fe and:
(a) Bxe2x80x3 is Co;
(b) Bxe2x80x3 is Co and 1.4xe2x89xa6y+yxe2x80x2 less than 2.0;
(c) Bxe2x80x3 is Co and 1.6xe2x89xa6y+yxe2x80x2 xe2x89xa61.9;
(d) Bxe2x80x3 is Co, 1.6xe2x89xa6y+yxe2x80x2xe2x89xa61.9, A is La, Axe2x80x3 is Sr and x+xxe2x80x2 is 2;
(e) Bxe2x80x3 is Mg;
(f) Bxe2x80x3 is Mg and 1.5xe2x89xa6y+yxe2x80x2xe2x89xa62.0;
(g) Bxe2x80x3 is Mg and 1.7xe2x89xa6y+yxe2x80x2xe2x89xa61.9;
(h) Bxe2x80x3 is Mg, 1.7xe2x89xa6y+yxe2x80x2xe2x89xa61.9, A is La, Axe2x80x3 is Sr and x+xxe2x80x2 is 2;
(i) Bxe2x80x3 is Mg, y+yxe2x80x2=18, A is La, Axe2x80x3 is Sr and x+xxe2x80x2 is 2;
(j) B is Ga, Bxe2x80x3 is Co, 1.6xe2x89xa6y+yxe2x80x2xe2x89xa61.9, A is La, Axe2x80x3 is Sr and x+x is 2;
(k) B is Ga, Bxe2x80x3 is Mg, 1.7xe2x89xa6y+yxe2x80x2xe2x89xa61.9, A is La, Axe2x80x3 is Sr and x+xxe2x80x2 is 2;
(l) B is Ga, y is 0.3 to 0.9, Bxe2x80x3 is Co, 1.6xe2x89xa6y+yxe2x80x2xe2x89xa61.9, A is La, Axe2x80x3 is Sr and x+xxe2x80x2 is 2;
(m) B is Ga, y is 0.3 to 0.9, Bxe2x80x3 is Mg, 1.7xe2x89xa6y+yxe2x80x2xe2x89xa61.9, A is La, Axe2x80x3 is Sr and x+xxe2x80x2 is 2;
(n) B is Al, Bxe2x80x3 is Co, 1.6xe2x89xa6y+yxe2x80x2xe2x89xa61.9, A is La, Axe2x80x3 is Sr and x+xxe2x80x2 is 2;
(o) B is Al, Bxe2x80x3 is Mg, 1.7xe2x89xa6y+yxe2x80x2xe2x89xa61.9, A is La, Axe2x80x3 is Sr and x+xxe2x80x2 is 2;
(p) B is Al, y is 0.3 to 0.9, Bxe2x80x3 is Co, 1.6xe2x89xa6y+yxe2x80x2xe2x89xa61.9, A is La, Axe2x80x3 is Sr and x+xxe2x80x2 is 2; or
(q) B is Al ,y is 0.3 to 0.9, Bxe2x80x3 is Mg, 1.7xe2x89xa6y+yxe2x80x2xe2x89xa61.9, A is La, Axe2x80x3 is Sr and x+xxe2x80x2 is 2.
Electronically- and ionically-conducting membranes employed in the oxygen-separation reactors of this invention comprise mixed metal oxides of the above formula. Substantially gas-impermeable membranes having both electronic and ionic conductivity are formed by initially preparing mixed metal oxide powders by repeatedly calcining and milling the powders of individual metal oxides or the corresponding carbonates (or other metal precursors) in the desired stoichiometric ratios. The resulting mixed metal oxide is then pressed and sintered into dense membranes of various shapes, including disks and open-one-ended tubes. These membranes are then employed to construct catalytic membrane reactors, particularly for oxygen separation processes. The purity of the product oxygen produced in reactors of this invention, which can be stored or used in other chemical processes, is generally greater than about 90% and preferably greater than about 99%.
The presence of the mixed metal oxide of desired stoichiometry (as in the given formulas) in a repeatedly calcined and milled mixed metal oxide can be assessed by X-ray diffraction studies. Further, the presence of distinct phases of metal oxides or other metal species that may be present in the mixed metal oxides materials of this invention can be detected by X-ray diffraction techniques by the observation of peaks not assignable with the predominate mixed metal oxide of desired stoichiometry. The level of distinct phase material that can be detected depends upon the resolution and sensitivity of the X-ray diffractometer employed and upon the identity and number of the distinct phases present. It is believed that greater than about 4% by weight of another phase can be detected by the X-ray diffraction method employed (FIGS. 1-6).
A catalytic reactor of this invention comprises an oxidation zone and a reduction zone separated by the substantially gas-impermeable catalytic membrane which comprises the electronically and ionically conducting mixed metal oxides of the above formula. Once in the reactor, the membrane has an oxidation surface in contact with the oxidation zone of the reactor and a reduction surface in contact with the reduction zone of the reactor. Electronic conduction in the reactor is provided through the membrane material which is a mixed ion and electron conductor (i.e., conducts both electrons and ions, such as oxygen anions). A reactor also comprises passages for admission of oxygen-containing gas, such as air, into the reactor reduction zone and admission of an oxygen-depleted gas, inert gas or reduced gas into the oxidation zone of the reactor. A vacuum can alternatively be applied to the oxidation zone to remove separated oxygen from the oxidation zone. Oxygen removed in this way can be collected and concentrated, if desired. The reactor also has gas exit passages from the reduction and oxidation zones. A plurality of membrane reactors can be provided in series or in parallel (with respect to gas flow through the reactor) to form a multi-membrane reactor to enhance speed or efficiency of oxygen separation.
In operation for oxygen separation, an oxygen-containing gas, such as air, is introduced into the reduction zone of the reactor in contact with the reduction surface to the catalytic membrane. Oxygen is reduced to oxygen anion at the reduction surface and the anion is conducted through the membrane to the oxidation surface. At the oxidation surface, the oxygen anion is re-oxidized to oxygen which is released into the oxidation zone of the reactor. (Alternatively, oxygen anion can be employed to oxidize a reduced gas (e.g., a hydrocarbon gas) at the oxidation surface of the membrane.) Membrane materials of this invention conduct electrons as well as anions. (Membrane materials that also conduct electrons allow charge neutralization of the membrane during operation.) Gases in the reactor can be under ambient or atmospheric pressure or they can be placed under higher or lower pressure (e.g., a vacuum can be applied) than ambient conditions. During operation for oxygen separation, the membrane is heated typically at a temperature above about 700xc2x0 C. and more typically from about 700xc2x0 C. to about 1100xc2x0 C. Preferred materials of this invention can be efficiently operated at temperatures that are generally lower than those currently used in the art, at from about 700xc2x0 C. to about 900xc2x0 C.
The oxidation surface, or the reduction surface or both surfaces (or parts of those surfaces) of the membrane can be coated with an oxidation catalyst or reduction catalyst, respectively, or both. A preferred catalyst for either or both surfaces of the membrane is La0.8Sr0.2CoO3xe2x88x92z where z is a number that makes the oxide charge neutral.
An oxygen flux of about 1 ml/min-cm2 or higher can be obtained through a 1 mm-thick membrane at ambient pressure and at an operating temperature of about 900xc2x0 C. These high flux rates can be maintained for long-term operation, e.g., up to about 700 h of operation.
Membrane materials as described herein can be employed in a method for oxygen separation from an oxygen-containing gas. In this method a reactor, as described above, is provided with a substantially gas-impermeable membrane which separates an oxidation and reduction zone. Oxygen is reduced at the reducing surface of the membrane. The resulting oxygen anions are then transported across the membrane to the reduction surface where oxygen anions are re-oxidized to form oxygen which is released into the oxidation zone for collection. This method can be employed to generate high purity oxygen (greater than about 90% purity) or very high purity oxygen (greater than about 99% purity) or to generate oxygen-enriched gases (e.g., oxygen in an inert gas).